A solar collector is a device for extracting the energy of the sun directly into a more usable or storable form. The energy in sunlight is in the form of electromagnetic radiation from the infrared (long) to the ultraviolet (short) wavelengths. The solar energy striking the earth's surface at any one time depends on weather conditions, as well as location and orientation of the surface, but overall, it averages about 1000 watts per square meter on a clear day with the surface directly perpendicular to the sun's rays.
A solar thermal collector that stores heat energy is called a “batch” type system. Other types of solar thermal collectors do not store energy but instead use fluid circulation (usually water or antifreeze solutions or refrigerants) to transfer the heat for direct use or storage in an insulated reservoir. Heat transfer medium/glycol has a high thermal capacity and is therefore convenient to handle. The direct radiation is captured using a dark colored surface which absorbs the radiation as heat and conducts it to the transfer fluid. Metal makes a good thermal conductor, especially copper and aluminum.
The fluid carries away the absorbed heat, thus cooling the absorber. The warmed fluid leaving the collector is either directly stored, or else passes through a heat exchanger to warm another tank of heat transfer medium, or is used to heat a building directly.
The transfer of heat is normally from a high temperature object to a lower temperature object. Classical transfer of thermal energy occurs only through conduction, convection, radiation or any combination of these. Conduction can be defined as a transfer of heat by electron diffusion or phonon vibrations, convection as a transfer of heat by conduction in a moving medium, such as a fluid, and radiation as a transfer of heat by electromagnetic radiation or, equivalently, by photons.
Conduction is heat transfer without any motion of the material as a whole. If one end of a metal rod is at a higher temperature, then energy will be transferred from a hot part of a body toward a colder one because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones. For heat transfer between two plane surfaces, such as heat loss through the wall of a house, the rate of conduction heat transfer is:
      Q    t    =            kA      ⁡              (                              T            hot                    -                      T            cold                          )              d  Q is heat transferred in time t, k is thermal conductivity of a thermal conductor, A is an area of a body cross section, Thot and Thot are temperatures of hot and cold edges of the body, respectively, d is a distance between hot and cold edges.
The design concept used for heating and cooling systems is similar. Materials with high thermal conductivity are used in both systems.
U.S. Pat. No. 4,459,976 discloses a heat exchanger/solar collector. An absorbing element is a thin radiation-absorbing layer composed of particles of crystalline graphite in light-transparent plate-type container. Heat-removing results from heat transfer medium flowing between particles of crystalline graphite.
US Application 2007/0158050 teaches a microchannel heat sink made of graphite material. A heat sink member has a plurality of microchannels formed therein for carrying heat transfer medium heat transfer medium. The each microchannel has a length parallel to outer surfaces of the heat sink member.
These two technical solutions have similar disadvantages in terms of application of them in solar heat collecting devices. The solar heat collector taught in '976, requires quite powerful heat removal (in terms of watts) and consequently large heat transfer medium flow. The heat sink solution taught in '050 suggested for smaller bodies such as electronic components uses microchannels to cool the body. Microchannels provide, however high resistance to fluid flow. Thus both of the above mentioned technical solutions are not optimally effective for use in solar heat collecting devices. Additionally, the microchannel devices are likely to fail in long-term usage because they will become clogged by dissolved salts in circulating heat transfer medium.
Graphite sheets are a potentially useful material, due to their anisotropic properties, specifically anisotropic thermal conductivity. Thermal conductivity of graphite sheets in a direction parallel to the crystallographic plane is several-fold higher in comparison with other directions.
Anisotropy in thermal conductivity of graphite sheets is used in a heat sink made of longer and shorter graphite sheets, see U.S. Pat. No. 6,771,502. A heat sink apparatus comprises alternating longer and shorter sheets of graphite material sandwiched together such that the longer sheets extend beyond the shorter sheets to define fins. The directions of higher thermal conductivity of the anisotropic graphite material are oriented in the plane of the sheet. The longer and shorter sheets have base ends aligned together to define a generally planar base surface for engaging an electronic device to be cooled.
The heat sink cooling taught in '502 is implemented by air convection which is the least effective type of heat transfer. Additionally, graphite sheets are attached to the cooled surface by means of end face surfaces of graphite sheets only. Enhancement of the area of thermal contact would facilitate an increased heat transfer rate.
An efficient means and method of extraction of converted heat from solar collecting devices is hence still a long felt need.